<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
    let count = 0;
    let h = document.querySelector("h1");
    //开启定时器
    setInterval(f,1000);
    function f() {
        // let date = new Date();
        // let mi = date.getTime();
        count++;
        h.innerText=count;
    }

    //匿名方法开启定时器
    let timer = setInterval(function (){
        console.log(count);
        if (count==5){
            //停止定时器
            clearInterval(timer);
        }
    },1000);

    //开启只执行一次的定时器
    setTimeout(function (){
        alert("时间到!")
    },3000)
</script>
</body>
</html>